An organelle is nothing but an organ of the cell. Traditionally, only the cell organs that are bound by individual membrane are called organelles. However, we may also refer to some non-bound organs as organelles. Examples of eukaryotic cell organelles are the endoplasmic reticulum, chloroplast, mitochondria, Golgi apparatus, vacuole, nucleus, flagellum, etc. Prokaryotic cells are structurally much simpler as compared to eukaryotic cells and hence contain much smaller number of organelle. Some examples of prokaryotic cell organelle are flagellum, pilus (plural, pili), plasmid, ribosome, nucleoid, mesosome, etc.
Organelles are responsible for specialized functions within a cell. For example, the chloroplast contains chlorophyll (in plant cells) and is responsible for the process of photosynthesis. Similarly, the mitochondria is the power house of the cell and generates ATP molecules from substrate.
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